Gas explosions inside mines are often caused when a flame or electrical spark ignites gas leaking from the coal seam

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A map of China's Chongqing region, where at least 15 miners are dead and another 18 are still missing.

Hundreds of rescuers Tuesday were struggling to find 18 coal miners still trapped a day after a gas explosion killed 15 of their colleagues in western China.

Whether the 18 are alive was not known more than 24 hours after the blast ripped through the privately owned Jinshangou mine in the sprawling Chongqing region. Just two miners were confirmed to have survived the blast.

Thirteen fatalities had been confirmed Tuesday morning. Later, two more bodies were recovered from among the 33 left in the shaft following the explosion, Xinhua News Agency reported.

The searchers were being hindered by debris blocking some of the mine's passageways.

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We "will exert our utmost as long as there's still a ray of hope," Chongqing deputy mayor Mu Huaping said of the search efforts, according to Xinhua.

Local officials did not answer telephone calls from The Associated Press, and a person who answered the phone at the mine hung up when asked about the blast.

Gas explosions inside mines are often caused when a flame or electrical spark ignites gas leaking from the coal seam. Ventilation systems are supposed to prevent gas from becoming trapped.

The State Administration of Work Safety ordered an investigation into the blast, "adding that those responsible must be strictly punished." Local officials in Chongqing also ordered smaller mines to shut down temporarily, Xinhua said.

China's mining industry has long been among the world's deadliest. The head of China's State Administration of Work Safety said earlier this year that struggling coal mines might be likely to overlook maintenance.

China is the world's largest producer and consumer of coal but plans to shutter more than 1,000 outdated mines, as part of a broader plan to reduce overproduction.